In the past, many types and arrangements of seals have been used in many different environments and equipment to provide a thermal barrier between different temperature fluids in adjacent regions. Examples of such seals include plate seals, membrane seals, metal strip seals, labyrinth seals, and leaf seals.
One type of equipment where seals are used to provide a thermal barrier is an air preheater used to recover heat from hot exhaust gas to increase thermal efficiency. One type of air preheater commonly used with industrial boilers is a regenerative air heater to preheat air entering a boiler with heat recovered flue gases exiting the boiler increasing efficiency and reducing CO2 emissions. Regenerative air heaters are commonly used in boilers in coal fired power plants throughout the world. (as well as fluidized bed boilers, waste-to-energy incinerators, and in hydrocarbon processing, steel mills, pulp & paper plants, chemical plants and other heavy industries.)
A regenerative air heater, also known as a preheater, has a circular rotor that rotates relatively slowly (about 3-5 rpm) within a heater casing during operation. The most common type is of the Ljungström design where the rotor is divided into a number of sectors each containing a heat absorbing medium. As the rotor rotates, the heat absorbing medium in each sector alternately recovers heat from hot flue gases exiting the boiler and transfers the recovered heat to cooler intake air entering the boiler preheating the air.
In the past, removable metal strip seals have been used between each sector and the preheater casing to prevent flue gas leakage. As the rotor rotates, the metal strip seals wear as a result of frictional contact between their outer edge and the surface against which they are intended to seal. Due to temperature changes in the preheater, thermal expansion causes the size of the gaps to fluctuate. The seal wears to the smallest gap size, leaving the larger gaps sizes unsealed. These metal strip seals are also subject to degradation from corrosion as well as erosion due to the relatively corrosive high temperature, high air flow atmosphere in which they operate. Additionally, if the metal strip seals are installed improperly, of there is an unusually large deviation in the predicted gap, the seals can completely break-off or they can cause the rotor to stop, forcing an unscheduled outage. Because the seals can only be replaced when the preheater is offline, replacement is infrequent as it can only be done during regularly scheduled downtime for boiler servicing.
Since boiler downtime is extremely costly, scheduled downtime is limited to only once or twice per year, and in some cases once every three years. The metal strip seals often degrade substantially during the normal operating cycle, resulting in seal leakage increasing to an unacceptable level well before scheduled downtime occurs. Flue gas leakage reduces heat transfer to air entering the boiler, which reduces boiler efficiency. Leakage of flue gas into the air entering the boiler not only further reduces boiler efficiency but it also undesirably increases boiler CO2 emissions due to increased fuel consumption. Air leakage causes the induced-draft and forced draft fans to work harder consuming more energy. Air heater leakage also affects the performance of air pollution control equipment due to impact of inconsistent temperatures on airflow, densities, catalytic reactions and corrosion downstream.
Brush seals have also been used in the past that employ metal brush bristles. While brush seals have proven superior to metal strip seals in reliability due to their flexibility and wear resilience, their permeable nature still allows undesirable leakage such that this type of seal has not been believed to heretofore been adopted in regenerative air preheaters.
One type of brush seal often used in turbine and turbo-equipped machinery applications includes a plate-type membrane that can be positioned within the bristles transversely or longitudinally relative to the brush seal. Even though such membranes have been disclosed to be flexible, they are considerably thicker than brush bristle diameter in order to support the bristles during brush seal operation. Unless seals of this type are in an annular or inverted ring configuration, where they are typically used to seal and/or mate with a shaft, axle or rotor and move in parallel with the plane of the seal, the membrane is prone to many of the same disadvantages as metal strip seals as they are subject to high wear and permanent deformation or breakage under stress (especially when they are placed exterior to the brush).
What is needed is an improved seal that provides the flexibility and long life reliability of brush combined with impermeability of a plate or membrane barrier that will deliver effective, consistent sealing over several plant maintenance cycles.